ULTIMATE LIMITS OF CCD PERFORMANCE IMPOSED BY HOT-ELECTRON EFFECTS

It is shown that the ultimate speed of charge transfer in silicon charge coupled devices is limited by impact ionization in the silicon and the saturation of the drift velocity. The charge transfer speed of buried channel devices (BCCDs) decreases linearly with the separation, L, between centers of neighboring gates for L ≤ 10 μM. The decrease of the transfer speed of surface channel devices (SCCDs) is linear only for L ≤ 2 μm but is much faster for L > 2 μm. For L ≤ 2 μm, the ultimate charge transfer times of both SCCDs and BCCDs are about the same, corresponding to clock frequencies in the GHz range. Charge coupled devices made in small-energy gap compound semiconductors are also discussed. In spite of the higher carrier mobility, devices in these materials can not be operated at clock frequency above the KHz-MHz range due to interband impact generation of electron-hole pairs in the high electric field. © 1979.